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      操作系统期末考试——3
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    <h2 id="局部性原理"><a href="#局部性原理" class="headerlink" title="局部性原理"></a>局部性原理</h2><p>一个程序的执行往往呈现出局部性规律，即在一段较短时间内，程序的执行仅局限于某个部分，它所访问的存储空间也局限于某个区域。<br>局限性表现在时间与空间两方面：</p>
<ul>
<li><p><strong>时间局限性</strong><br>如果程序中的某条指令一旦执行， 则不久以后该指令可能再次执行；如果某数据被访问过， 则不久以后该数据可能再次被访问，典型原因是由于程序中循环操作。</p>
</li>
<li><p><strong>空间局限性</strong><br>一旦程序访问了某个存储单元，在不久之后，其附近的存储单元也将被访问，即程序在一段时间内所访问的地址，可能集中在一定的范围之内，典型原因是程序的顺序执行。</p>
</li>
</ul>
<h2 id="虚拟存储器定义及特征"><a href="#虚拟存储器定义及特征" class="headerlink" title="虚拟存储器定义及特征"></a>虚拟存储器定义及特征</h2><h3 id="定义"><a href="#定义" class="headerlink" title="定义"></a>定义</h3><p>具有请求调入功能和置换功能， 能从逻辑上对内存容量加以扩充的一种存储器系统。<br>其逻辑容量由内存容量和外存容量之和所决定，其运行速度接近于内存速度，而每位的成本却又接近于外存。<br>虚拟存储技术是一种性能非常优越的存储器管理技术，故被广泛地应用于大、 中、 小型机器和微型机中。</p>
<h3 id="特征"><a href="#特征" class="headerlink" title="特征"></a>特征</h3><p><strong>多次性</strong> ：一个作业被分成多次调入内存运行；<br><strong>对换性</strong> ： 允许在作业的运行过程中换出，换进 ；<br><strong>虚拟性</strong> ：指从逻辑上扩充内存容量，使用户所看到的内存容量远大于实际内存容量。</p>
<h2 id="虚拟存储器的实现硬件支持是什么？"><a href="#虚拟存储器的实现硬件支持是什么？" class="headerlink" title="虚拟存储器的实现硬件支持是什么？"></a>虚拟存储器的实现硬件支持是什么？</h2><h3 id="请求分页系统"><a href="#请求分页系统" class="headerlink" title="请求分页系统"></a>请求分页系统</h3><p>置换是以页为单位进行</p>
<h4 id="硬件支持"><a href="#硬件支持" class="headerlink" title="硬件支持"></a>硬件支持</h4><p><strong>页表</strong></p>
<p><img  srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20300%20300'%3E%3C/svg%3E" data-src="https://api2.mubu.com/v3/document_image/3171dfb0-c676-4577-85c2-0052a080245c-3807603.jpg" class="lozad post-image"src="https://api2.mubu.com/v3/document_image/3171dfb0-c676-4577-85c2-0052a080245c-3807603.jpg"></p>
<p><strong>缺页中断机构</strong></p>
<ul>
<li>在地址映射过程中，在页表中发现所要访问的页不在内存，则产生缺页中断；</li>
<li>操作系统接到此中断信号后，就调出缺页中断处理程序，根据页表中给出的外存地址，将该页调入内存；</li>
<li>如果内存中有空闲块，则分配一页，将新调入页装入内存，并修改页表中相应页表项目的状态位及相应的物理块号；</li>
<li>若此时内存中没有空闲块，则要淘汰某页，若该页在内存期间被修改过，则要将其写回外存。</li>
</ul>
<p><strong>地址变换机构</strong></p>
<p><img  srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20300%20300'%3E%3C/svg%3E" data-src="https://api2.mubu.com/v3/document_image/2f0802fc-45b4-4ba5-a623-4fbee46b4371-3807603.jpg" class="lozad post-image"src="https://api2.mubu.com/v3/document_image/2f0802fc-45b4-4ba5-a623-4fbee46b4371-3807603.jpg"></p>
<h4 id="软件支持"><a href="#软件支持" class="headerlink" title="软件支持"></a>软件支持</h4><p><strong>请求调页</strong><br>进程运行装入部分页面；之后根据进程运行需要，动态装入其它页面。</p>
<p><strong>页面置换</strong><br>当内存空间已满，需要装入新的页面时，则根据某种算法淘汰某个页面；装入新的页面。</p>
<h3 id="请求分段系统"><a href="#请求分段系统" class="headerlink" title="请求分段系统"></a>请求分段系统</h3><p>置换是以段为单位进行</p>
<h4 id="硬件支持-1"><a href="#硬件支持-1" class="headerlink" title="硬件支持"></a>硬件支持</h4><ul>
<li>段表</li>
<li>缺段中断机构</li>
<li>地址变换机构</li>
</ul>
<h4 id="软件支持-1"><a href="#软件支持-1" class="headerlink" title="软件支持"></a>软件支持</h4><p>请求调段，分段置换</p>
<h2 id="缺页中断与一般中断的区别是什么？"><a href="#缺页中断与一般中断的区别是什么？" class="headerlink" title="缺页中断与一般中断的区别是什么？"></a>缺页中断与一般中断的区别是什么？</h2><h3 id="相同点"><a href="#相同点" class="headerlink" title="相同点"></a>相同点</h3><ul>
<li>需要保护CPU环境</li>
<li>分析中断原因</li>
<li>转入缺页中断处理程序</li>
<li>恢复CPU环境。</li>
</ul>
<h3 id="不同点"><a href="#不同点" class="headerlink" title="不同点"></a>不同点</h3><ul>
<li>CPU都是在一条指令执行完后，才检查是否有中断请求到达；缺页中断是在指令执行期间产生和处理中断信号。</li>
<li>一条指令在执行期间可能产生多次缺页中断。</li>
</ul>
<h2 id="什么是抖动现象？产生抖动的原因是什么？"><a href="#什么是抖动现象？产生抖动的原因是什么？" class="headerlink" title="什么是抖动现象？产生抖动的原因是什么？"></a>什么是抖动现象？产生抖动的原因是什么？</h2><p>即刚被换出的页很快又要被访问，需要将它重新调入，此时又需要再选一页调出。而此刚被调出的页很快又被访问，又需将它调入，如此频繁更换页面，以致一个进程在运行中把大部分时间都花费在页面置换上，我们称该进程发生了“抖动” 。</p>
<h3 id="现象"><a href="#现象" class="headerlink" title="现象"></a>现象</h3><p>请求分页系统中，页面在内存与外存之间频繁调度，处理器花费大量的时间用于对换页面而不是执行计算任务的现象。<br>发生抖动现象后，系统效率急剧下降，甚至导致系统崩溃。</p>
<h3 id="原因"><a href="#原因" class="headerlink" title="原因"></a>原因</h3><ul>
<li>页面置换算法不合理</li>
<li>分配给进程的物理页面数太少</li>
</ul>
<h2 id="页面置换算法"><a href="#页面置换算法" class="headerlink" title="页面置换算法"></a>页面置换算法</h2><h3 id="最佳置换-OPT"><a href="#最佳置换-OPT" class="headerlink" title="最佳置换 OPT"></a>最佳置换 OPT</h3><p>算法选择的被淘汰页面：将是以后永不使用的或是在最长(未来)时间内不再被访问的页面。通常可保证获得最低的缺页率</p>
<p><img  srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20300%20300'%3E%3C/svg%3E" data-src="https://api2.mubu.com/v3/document_image/d47a5a4e-9d36-4298-8332-29f44c0f9cd9-3807603.jpg" class="lozad post-image"src="https://api2.mubu.com/v3/document_image/d47a5a4e-9d36-4298-8332-29f44c0f9cd9-3807603.jpg"></p>
<h3 id="先进先出-FIFO"><a href="#先进先出-FIFO" class="headerlink" title="先进先出 FIFO"></a>先进先出 FIFO</h3><p>该算法淘汰最先进入内存的页面（或驻留内存最久的页面）</p>
<p><img  srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20300%20300'%3E%3C/svg%3E" data-src="https://api2.mubu.com/v3/document_image/806f8968-c0e4-45da-8ffb-12e57b2db4b4-3807603.jpg" class="lozad post-image"src="https://api2.mubu.com/v3/document_image/806f8968-c0e4-45da-8ffb-12e57b2db4b4-3807603.jpg"></p>
<h3 id="最近最久未使用-LRU"><a href="#最近最久未使用-LRU" class="headerlink" title="最近最久未使用 LRU"></a>最近最久未使用 LRU</h3><p>选择最后一次访问时间距离当前时间最长的一页(最近最久未使用）并淘汰之，即淘汰最近最久未使用的页。</p>
<p><img  srcset="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%20300%20300'%3E%3C/svg%3E" data-src="https://api2.mubu.com/v3/document_image/99950127-bf64-4cc1-bf2b-2f612037db9e-3807603.jpg" class="lozad post-image"src="https://api2.mubu.com/v3/document_image/99950127-bf64-4cc1-bf2b-2f612037db9e-3807603.jpg"></p>
<h2 id="缺页中断次数"><a href="#缺页中断次数" class="headerlink" title="缺页中断次数"></a>缺页中断次数</h2><p>缺页率 = 缺页中断次数/访问页面总数</p>
<h2 id="页面置换次数"><a href="#页面置换次数" class="headerlink" title="页面置换次数"></a>页面置换次数</h2><p>置换率=页面置换次数/访问页面总数</p>
<h2 id="物理块分配算法"><a href="#物理块分配算法" class="headerlink" title="物理块分配算法"></a>物理块分配算法</h2><p>平均分配算法<br>按比例分配算法<br>考虑优先权的分配算法</p>

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<ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%B1%80%E9%83%A8%E6%80%A7%E5%8E%9F%E7%90%86"><span class="toc-text">局部性原理</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E8%99%9A%E6%8B%9F%E5%AD%98%E5%82%A8%E5%99%A8%E5%AE%9A%E4%B9%89%E5%8F%8A%E7%89%B9%E5%BE%81"><span class="toc-text">虚拟存储器定义及特征</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%AE%9A%E4%B9%89"><span class="toc-text">定义</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E7%89%B9%E5%BE%81"><span class="toc-text">特征</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E8%99%9A%E6%8B%9F%E5%AD%98%E5%82%A8%E5%99%A8%E7%9A%84%E5%AE%9E%E7%8E%B0%E7%A1%AC%E4%BB%B6%E6%94%AF%E6%8C%81%E6%98%AF%E4%BB%80%E4%B9%88%EF%BC%9F"><span class="toc-text">虚拟存储器的实现硬件支持是什么？</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E8%AF%B7%E6%B1%82%E5%88%86%E9%A1%B5%E7%B3%BB%E7%BB%9F"><span class="toc-text">请求分页系统</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#%E7%A1%AC%E4%BB%B6%E6%94%AF%E6%8C%81"><span class="toc-text">硬件支持</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E8%BD%AF%E4%BB%B6%E6%94%AF%E6%8C%81"><span class="toc-text">软件支持</span></a></li></ol></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E8%AF%B7%E6%B1%82%E5%88%86%E6%AE%B5%E7%B3%BB%E7%BB%9F"><span class="toc-text">请求分段系统</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#%E7%A1%AC%E4%BB%B6%E6%94%AF%E6%8C%81-1"><span class="toc-text">硬件支持</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E8%BD%AF%E4%BB%B6%E6%94%AF%E6%8C%81-1"><span class="toc-text">软件支持</span></a></li></ol></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E7%BC%BA%E9%A1%B5%E4%B8%AD%E6%96%AD%E4%B8%8E%E4%B8%80%E8%88%AC%E4%B8%AD%E6%96%AD%E7%9A%84%E5%8C%BA%E5%88%AB%E6%98%AF%E4%BB%80%E4%B9%88%EF%BC%9F"><span class="toc-text">缺页中断与一般中断的区别是什么？</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E7%9B%B8%E5%90%8C%E7%82%B9"><span class="toc-text">相同点</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E4%B8%8D%E5%90%8C%E7%82%B9"><span class="toc-text">不同点</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E4%BB%80%E4%B9%88%E6%98%AF%E6%8A%96%E5%8A%A8%E7%8E%B0%E8%B1%A1%EF%BC%9F%E4%BA%A7%E7%94%9F%E6%8A%96%E5%8A%A8%E7%9A%84%E5%8E%9F%E5%9B%A0%E6%98%AF%E4%BB%80%E4%B9%88%EF%BC%9F"><span class="toc-text">什么是抖动现象？产生抖动的原因是什么？</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E7%8E%B0%E8%B1%A1"><span class="toc-text">现象</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%8E%9F%E5%9B%A0"><span class="toc-text">原因</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E9%A1%B5%E9%9D%A2%E7%BD%AE%E6%8D%A2%E7%AE%97%E6%B3%95"><span class="toc-text">页面置换算法</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E6%9C%80%E4%BD%B3%E7%BD%AE%E6%8D%A2-OPT"><span class="toc-text">最佳置换 OPT</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%85%88%E8%BF%9B%E5%85%88%E5%87%BA-FIFO"><span class="toc-text">先进先出 FIFO</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E6%9C%80%E8%BF%91%E6%9C%80%E4%B9%85%E6%9C%AA%E4%BD%BF%E7%94%A8-LRU"><span class="toc-text">最近最久未使用 LRU</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E7%BC%BA%E9%A1%B5%E4%B8%AD%E6%96%AD%E6%AC%A1%E6%95%B0"><span class="toc-text">缺页中断次数</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E9%A1%B5%E9%9D%A2%E7%BD%AE%E6%8D%A2%E6%AC%A1%E6%95%B0"><span class="toc-text">页面置换次数</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E7%89%A9%E7%90%86%E5%9D%97%E5%88%86%E9%85%8D%E7%AE%97%E6%B3%95"><span class="toc-text">物理块分配算法</span></a></li></ol>
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